The present invention is an improved susceptor for a float-zone apparatus for the float-zone processing of silicon elements. The susceptor is of a cylindrical design which allows the susceptor to be positioned around a free-end of a silicon element to heat the free-end of the silicon element to facilitate inductive coupling of the free-end of the silicon element with an RF induction coil heater. In a is preferred embodiment of the present invention, the susceptor is formed from tantalum.
Apparatuses for the float-zone processing of silicon are well known in the art and are described for example in British Patent No. 1,081,827, published Sep. 6, 1967, and in Burd et al., U.S. Pat. No. 4,045,181, issued Aug. 30, 1977, both of which are incorporated by reference herein. Hwang et al., co-pending U.S. patent application Ser. No. 07/942,907, filed Sep. 10, 1992, describes a method of analyzing chunked silicon for contaminates, where the chunked silicon is float-zone processed to effect distribution of contaminates into the bulk of a monocrystal of silicon. In addition, apparatuses and procedures for the float-zone processing of silicon are described extensively in Keller et al., Preparation and Properties of Solid State Materials: Floating-Zone Silicon, Vol. 5, Marcel Dekker, Inc., New York, N.Y., 1981.
Standard procedures for the float-zone processing of silicon employ a radio frequency (RF) induction coil heater to create a melt zone through a cross-section of a silicon element. This melt zone can then be caused to move along the length of the silicon element to provide for refinement of the silicon element and effect formation of the silicon element into a monocrystal of silicon.
It is known that the amount of RF energy which is necessary to coupled a RF induction coil heater with a silicon element is inversely proportional to the resistivity of the silicon element. Therefore because relatively pure silicon, such as semiconductor grade silicon, has high resistivity at ambient temperature it is difficult to start the inductive heating of the silicon element. To overcome this resistance to inductive heating, float-zone apparatuses typically contain a susceptor which couples with the RF induction coil heater more readily than the silicon element and provides radiant heat to the silicon element to decrease its resistivity. These susceptors are typically solid carbon or graphite elements which are positioned in close proximity to the silicon element, but do not surround a free end of the silicon element.
Therefore, it is an objective of the present invention to provide an improved apparatus for the float-zone processing of silicon, the improvement comprising a cylindrical susceptor positionable around the free end of a silicon element to effect preheating of the free end of the silicon element and facilitate inductive coupling of the free end of the silicon element with an RF induction coil heater. The cylindrical susceptor when positioned around the free end of a silicon element and heated by RF coupling with the RF induction coil heater of a float-zone apparatus provides more uniform and effective heating of the free end of the silicon element. The cylindrical susceptor is particularly useful for the pre-heating of irregular-shaped silicon elements such as described in Hwang, supra. In a preferred embodiment of the present invention the cylindrical susceptor is formed from tantalum.